Yes, this is the error I’ve noticed. Just had a quick look at 'mbstring.substitute_character'. Honestly, I have no idea which is better or worse 
It’s another issue. I’ve not seen it [yet].
i’m not sure if hiding this entirely is a good idea, blank html usually means a server error (which we can’t do anything about) but could also mean some problem with the tt-rss server which could be useful to report.